if var

If a variable is the condition of an if, inside the then branch the variable will be considered as not having the Nil type:

a = some_condition ? nil : 3
# a is Int32 or Nil

if a
  # Since the only way to get here is if a is truthy,
  # a can't be nil. So here a is Int32.
  a.abs
end

This also applies when a variable is assigned in an if's condition:

if a = some_expression
  # here a is not nil
end

This logic also applies if there are ands (&&) in the condition:

if a && b
  # here both a and b are guaranteed not to be Nil
end

Here, the right-hand side of the && expression is also guaranteed to have a as not Nil.

Of course, reassigning a variable inside the then branch makes that variable have a new type based on the expression assigned.

Limitations

The above logic works only for local variables. It doesn’t work with instance variables, class variables, or variables bound in a closure. The value of these kinds of variables could potentially be affected by another fiber after the condition was checked, rendering it nil. It also does not work with constants.

if @a
  # here `@a` can be nil
end

if @@a
  # here `@@a` can be nil
end

a = nil
closure = ->{ a = "foo" }

if a
  # here `a` can be nil
end

This can be circumvented by assigning the value to a new local variable:

if a = @a
  # here `a` can't be nil
end

Another option is to use Object#try found in the standard library which only executes the block if the value is not nil:

@a.try do |a|
  # here `a` can't be nil
end

Method calls

That logic also doesn't work with proc and method calls, including getters and properties, because nilable (or, more generally, union-typed) procs and methods aren't guaranteed to return the same more-specific type on two successive calls.

if method # first call to a method that can return Int32 or Nil
  # here we know that the first call did not return Nil
  method # second call can still return Int32 or Nil
end

The techniques described above for instance variables will also work for proc and method calls.